1. Field of the Invention
This invention relates to liquid crystal compounds and mixtures containing same. This invention also relates to the manufacture of these compounds and their use for electro-optical and chromatographic purposes.
2. Background Description
Liquid crystalline materials are of importance primarily as dielectrics in electro-optical indicating devices, since the optical properties of such materials can be influenced by an electric field. Electro-optical devices based on liquid crystals are well known to the person skilled in the art and can be based on various effects such as, for example, dynamic scattering, the deformation of aligned phases (DAP type), the Schadt-Helfrich effect (rotation cell), a cholesteric-nematic phase transition or the guest/host effect ("guest/host cell").
For a successful technical application of these effects it is, however, important that the materials used satisfy a number of additional requirements. For example, they must have a good chemical stability towards heat, moisture, air, electromagnetic radiation (in the ultraviolet, visible and infrared region), electric fields and the like, they must be colourless and they must give a good contrast when used in a cell. Further, they should have a low viscosity and short response times and should have a nematic or cholesteric mesophase in the entire temperature range in which the liquid crystal cell is to be operated. Other properties such as, for example, the dielectric anisotropy, the threshold potential and the conductivity must fulfil different conditions depending on the type of cell which is used.
Since, in general, it is not possible to achieve all of the desired and to some extent contradictory properties with a single compound, mixtures of two or more compounds are manufactured as a rule. In this case, however, it has to be taken into consideration that the components must not undergo chemical reactions with one another and also that they must have good miscibility at room temperature and lower temperatures. Further, the mixtures should have no smectic mesophases, at least at temperatures at which the liquid crystal cell is to be operated.
Liquid crystals with positive dielectric anisotropy (.DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp. &gt;0, .epsilon..sub..parallel. signifying the dielectric constant along the longitudinal molecular axis and .epsilon..sub..perp. signifying the dielectric constant perpendicular thereto) orientate themselves in an electric field with the direction of the largest dielectric constant, i.e. with the longitudinal molecular axis, parallel to the field direction. This effect is used, inter alia, in the aforementioned rotation cell and guest/host cell. In order to maintain a low threshold potential, the liquid crystal material used in this case should have a relatively high anisotropy of the dielectric constants. On the other hand, however, compounds with a large positive anisotropy of the dielectric constants generally have the disadvantage that at the same time they increase the viscosity of the liquid crystal material.